The ultrastructure and development of the sperm of sea cucumber Acaudina leucoprocta were observed by using scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM). As expected, spermatogenesis was divided into five periods: spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid and mature spermatozoon. Spermatogonia had the biggest volume. Chromatin condensation and a pair of centriole were found in spermatocyte. The mature spermatozoon belonged to primitive type, consisting of head, middle piece and tail. The head was spherical in shape with apical acrosome, and chromatin was condensation in the nucleus. Mitochondria complex and centrioles were observed in the middle piece, and the flagellum showed a simple "9+2" type.